When I shoot portraits indoors, I'd like the option to tether my Nikon D90 to a Windows computer with the USB cable, both to view the results and help adjust the lighting, but also to be making an instant backup of each shot.

Canon's software comes with this bundled in. Are you sure your camera did not come with a software disk that provides this functionality? Or have you tried Nikon's, and want an alternative?
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reuscamJul 16 '10 at 17:01

My understanding is this software is not included with the camera. Nikon sells software that includes this feature but it's over $100.
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jfklein13Jul 16 '10 at 17:34

Not for Windows, but I use Sofortbild on the Mac. It works nicely and it's free.
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Craig WalkerJul 16 '10 at 18:01

I've now used Camera Control from diyphotobits.com. It's OK; downloads a duplicate of the shots and shows a preview. What I wasn't anticipating is how slow the process would be, relatively speaking. I am using a somewhat slow laptop, and shooting in a RAW+JPG mode. It takes a couple of seconds just to download the ~15-20MB of data and render the preview.
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jfklein13Aug 11 '10 at 2:58